In a case where a direct-injection type fuel injection valve described above is used, typically, fuel is pressurized under high pressure by using a fuel pump and feedback control is performed so that a pressure of fuel (hereinafter simply referred to as a “fuel pressure”) fed to the fuel injection valve becomes a target fuel pressure set in accordance with actual rotational speed and load of an internal-combustion engine. Meanwhile, a fuel injection valve has a lower limit valve opening period, and a valve opening period of the fuel injection valve cannot be made shorter than the lower limit valve opening period because of valve opening characteristics of the fuel injection valve. Accordingly, in a case where a load and a target fuel pressure set in accordance with the load rapidly decrease, for example, during deceleration of an internal-combustion engine and where an actual fuel pressure does not follow the target fuel pressure well and does not decrease speedily, there is a possibility that an amount of fuel injection exceeds a requested amount of fuel and fuel is injected excessively in a state where a valve opening period of the fuel injection valve is set to a lower limit valve opening period. Hereinafter, this phenomenon is referred to as “sticking of an injection valve flow amount to a lower limit”.
For example, a device described in Japanese Unexamined Patent Application Publication No. 2007-154686 is known as a conventional fuel injection control device that is intended to overcome such a problem. In this device, an amount of fuel injection is calculated in accordance with a rotational speed of an internal-combustion engine and an amount of operation of an accelerator pedal, a target fuel pressure is calculated in accordance with the amount of fuel injection and the rotational speed of the internal-combustion engine, and feedback control is performed so that an actual fuel pressure becomes the target fuel pressure. In a fuel injection control device described in Japanese Unexamined Patent Application Publication No. 2016-156317, a change in load of an internal-combustion engine is predicted, and pressurizing operation of a fuel pump is stopped when it is predicted that the load decrease.
The inventors found that in the device described in Japanese Unexamined Patent Application Publication No. 2007-154686, a target fuel pressure is set in accordance with an amount of operation of an acceleration pedal, and it is therefore possible to speedily decrease an actual fuel pressure in accordance with the target fuel pressure during deceleration. However, for example, in a case where rapid deceleration is performed in the middle of an accelerated state, the actual fuel pressure has been already controlled to a high fuel pressure in accordance with the target fuel pressure in the accelerated state, and therefore the actual fuel pressure cannot be speedily decreased to a desired pressure even when the target fuel pressure is decreased from this state during deceleration. This leads to a risk of occurrence of the aforementioned sticking of an injection valve flow amount to a lower limit.
The inventors found that in the device described in Japanese Unexamined Patent Application Publication No. 2016-156317, pressurizing operation of a fuel pump is stopped when it is predicted that a load of an internal-combustion engine decrease, and it is therefore possible to speedily decrease a fuel pressure during deceleration. However, the fuel pressure cannot be controlled since the pressurizing operation of the fuel pump is completely stopped, and therefore there is a possibility that the fuel pressure decrease more than necessary. In such a case, there is a risk of failure to secure a fuel pressure necessary for re-acceleration after deceleration.
Thus, it is preferable to provide a fuel injection control device for an internal-combustion engine that can speedily decrease a fuel pressure in a right amount in accordance with a load of an internal-combustion engine during deceleration even in a case where deceleration is performed in the middle of an accelerated state, thereby preventing an injection valve flow amount from sticking to a lower limit.